Some operators give you all you can eat for X minutes. For example, Vodafone NL asks 0,75 EUR for 15 minutes with speed 3600/384 while T-Mobile NL costs by the 10 kB, rounds up, and provides less bandwidth. Vodafone NL would work great if you were to download some Google Maps for offline usage whereas on T-Mobile NL this would be expensive. Conversely, if you'd check your e-mail every hour or stay connected Vodafone NL's scheme would cost you a fortune.

The reason you use 100 kb of data is because of authentication with your e-mail server. If you stay connected some bytes will be used to keep the connection alive, and when it does check for messages it only has to sent that command every X minutes and receive reply hence skipping authentication. This will not work with POP3, but will work with IMAP4, and I suppose as well with something as Nokia Messaging. Also, I think IMAP-IDLE, which is supported on S60 client, reduces the need to check every X minutes further lowering data usage while allowing real-time communication when necessary. I don't know about P-IMAP though.